Question

I’m using the AFNetworking library to pull a JSON feed from a server to populate a UIPickerView, but I’m having a little trouble wrapping my head around the asynchronous way of doing things. The @property classChoices is an NSArray that’s being used to populate the UIPickerView, so that the web call is only performed once. However, since the block isn’t finished by the time the instance variable is returned, the getter returns nil, and it eventually causes my program to crash later on. Any help in fixing this would be greatly appreciated. Let me know if you need any additional information.

PickerViewController.m classChoices Getter

- (NSArray *)classChoices {
    if (!_classChoices) {
        // self.brain here refers to code for the SignUpPickerBrain below
        [self.brain classChoicesForSignUpWithBlock:^(NSArray *classChoices) {
            _classChoices = classChoices;
        }];
    }
    return _classChoices;
}

SignUpPickerBrain.m

- (NSArray *)classChoicesForSignUpWithBlock:(void (^)(NSArray *classChoices))block {
    [[UloopAPIClient sharedClient] getPath:@"mobClass.php" parameters:nil success:^(AFHTTPRequestOperation *operation, id responseJSON) {
        NSLog(responseJSON);
        if (block) {
            block(responseJSON);
        }
    } failure:^(AFHTTPRequestOperation *operation, NSError *error) {
        NSLog(@"Error: %@", error);

        if (block) {
            block(nil);
        }
    }];
}

Answer 1

You need a method like the following in your PickerViewController which returns the array once it has been downloaded. Once the callback has been returned, you can then continue on with your code:

- (void)classChoices:(void (^) (NSArray * classChoices)) _callback {
    if (!self.classChoices) {
        // self.brain here refers to code for the SignUpPickerBrain below
        [self.brain classChoicesForSignUpWithBlock:^(NSArray *classChoices) {
            _callback(classChoices);
        }];
    }
}

// call the method

- (void) viewDidLoad {

    [super viewDidLoad];

    [self classChoices:^(NSArray * updatedChoices) {

        self.classChoices = updatedChoices;

        [self.pickerView reloadAllComponents];

    }];

}